Zirconium tubing containing an outer layer of zirconium metal alloy and an inner layer of unalloyed zirconium metal is used extensively in nuclear power reactors and, in particular, in boiling water reactors.
The tubing is used to form a cladding to contain and support nuclear fuel pellets, usually made of uranium dioxide. The purpose of the pure or unalloyed zirconium liner is to reduce or prevent local chemical or mechanical interaction, or both, between the fuel pellets during the operation of the reactor and the more susceptible and more reactive outer zirconium alloy sheath. Such interactions between the fuel pellets and the cladding material is believed to be responsible for what is termed "iodine assisted stress corrosion cracking" of the outer zirconium alloy (Zircaloy) sheath. The resultant cracking of the sheath is deleterious to the safety of the reactor operation and to the lifetime of the fuel as it permits radioactive gaseous products of the fission reactions to diffuse therethrough and escape into the reactor vessel as well as permitting water or steam to contact the fuel elements directly.
The current accepted solution to the problem of iodine assisted stress corrosion cracking of zirconium alloys is the expedient of providing the structural zirconium alloy with an internal liner of substantially pure zirconium. This relatively inert unreactive liner provides the ductility required to prevent the pellet-cladding interactions described.
The success of such liners has prompted most manufacturers to specify pure or substantially pure zirconium liners for the cladding inner tube liner. As a consequence, lower levels of oxygen and iron impurities are being tolerated. This has created a secondary problem of major concern.
As zirconium is rendered purer, the metallurgical grain size of the zirconium in the liner tends to increase. Normally impurities such as iron when present in amounts above its solubility limit in zirconium tend to pin grain boundaries in place during the thermal processing required in the manufacture of the liner if the iron is present as a finely dispersed intermetallic second phase. Moreover, as the grain size increases, secondary grain growth occurs which contributes to the formation of a non-uniform bi-modal grain size distribution where many smaller grains co-exist with many larger grains. This bi-modal or duplex distribution creates problems during the subsequent fabrication processing for making barrier tube shells into finished tubing.
Normally a zirconium alloy tube mated to an unalloyed zirconium tube are tube reduced in a Pilger mill which reduces the size of the tube to the eventual size of the combination for its cladding function. When the purity of the zirconium liner has reduce the pining function of some impurities and a bi-modal grain distribution has formed, local microcracking begins to occur at the grain boundaries between the clusters of large and small grains. It is believed that the local deformation inhomogeneities present between clusters or aggregates of large grains and aggregates or clusters of small grains, causes the zirconium to respond differently to deformation induced straining. It appears that the stresses created in the tube reducing operation can exceed the cohesive strength of the grain boundaries. The resultant microcracks, if numerous or deep enough, will significantly reduce the liners ability to prevent the local pellet-cladding interactions previously described.
It is therefore an objective of the present invention to reduce the occurrence of microcracking at grain boundaries in relatively pure zirconium fuel cladding liner material.
It is a further objective of the present invention to produce uniformly sized relatively small grain sizes in zirconium cladding liner materials containing less than 300 parts per million of iron impurities.
It is a further object of the present invention to provide a method for preventing the formation of bi-modal grain size distributions in unalloyed zirconium to be used as fuel cladding liner material.
It is a further object of the present invention to provide a coextruded nuclear fuel cladding comprising an outer zirconium alloy tube bonded to an inner relatively pure unalloyed zirconium liner which can be fabricated by conventional mill practices and continue to exhibit superior resistance to deleterious fuel pellet cladding interactions.